Assuming that JKR wanted an all-Brit cast only because she wanted the accent right (not sure if this is true), I'd say we can ask actors to fake accents. I mean Elijah Wood, Liv Tyler, Brad Dourif etc managed quite well in LOTR, didn't they?

If that's true about accents, they still got it partially wrong seeing how the each member of the Weasley family seems to speak with a different type of British accent.